1. Technical Field
The present invention relates to a limited feedback method and apparatus for two-way relay channels with physical network coding, more particularly to a method and apparatus for providing phase information and power allocation information as feedback for two terminals with a wireless relay communicating bi-directionally.
2. Description of the Related Art
Wireless network coding has been a subject of intense research over the past several years in the area of cooperative relaying systems.
Recently, a two-way relaying system using physical network coding (PNC) was introduced.
As illustrated in FIGS. 1A and 1B, the protocol may be divided into a multiple access (MA) stage, in which the relay node (relay) 100 receives data simultaneously from two end nodes (a first terminal 102 and a second terminal 104), and a broadcast (BC) stage, in which the relay 100 simultaneously broadcasts the data received from the first terminal 102 to the second terminal 104 and the data received from the second terminal 104 to the first terminal 102.
Various systems for PNC have been proposed, some of which are as shown below.
S. Katti, H. Rahul, W. Hu, D. Katabi, and M. Medard, “XORs in the air: practical wireless network coding,” in Proc. Conf. Applications, Technol., Architect., and Protocols Comput. Commun., September 2006, introduces the Exclusive-OR (XOR) operation, which scheme entails a simple operation for PNC. This operation, however, suffers from performance degradation in the system due to noise from other users.
S. Zhang, S. C. Liew, and P. P. Lam, “Physical-Layer Network Coding,” in Proc. ACM MOBICOM, September 2006, considers the modulo operation instead of the XOR technique, in order to improve system performance. Assuming perfect synchronization, the modulo operation for PNC can achieve almost the same bit error rate (BER) performance as one-way systems with twice as much throughput.
Y.-T. Kim, M. Park, K.-J. Lee, and I. Lee, “Linear Precoding Design based on the Minimum Distance for Two-Way MIMO Physical Network Coding Systems,” in Proc. IEEE Globecom, December 2011, shows that employing the modulo operation and using precoding to make the channels of both terminals equal makes it possible to obtain optimal performance in terms of the minimum distance for a two-way relaying system.
T. Koike-Akino, P. Popovski, and V. Tarokh, “Optimized constellations for two-way wireless relaying with physical network coding,” IEEE Journal on Selected Areas in Communications, vol. 27, pp. 773-787, June 2009, proposes determining network coding functions and relay mappers for all instant channel conditions to maximize the minimum distance. The paper introduces a scheme in which the network coding functions and relay mappers selected by the relay are sent to the terminals by way of a limited number of feedback bits. Although this scheme exhibits good performance compared to the XOR system, there is the drawback that every node needs to know not only the numerous network coding functions and relay mappers but also very sophisticated selection criteria subject to the channel condition.
Considering the related art, it may be preferable to use a modulo operation for PNC in a two-way relaying system, but it is needed to lower the complexity compared to the existing limited feedback scheme.